Generally, in an orthogonal frequency division multiplexing (OFDM) systems, it is an effective method for adjusting a fast fourier transform (FFT) window with the use of protection intervals to use the channel impulse response (CIR) signal to noise ratio (SNR) information for adjusting the FFT window before transforming received data with the use of FFT, so as to minimize inter-symbol interference (ISI), inter-channel interference (ICI) and noise power, thereby improving the system performance.
However, in the conventional OFDM system, only the square FFT window or only portions thereof which is not damaged even using the guard interval is used. Thus, the longer a maximum delay length of the channel is, the more the limited gain is.
FIG. 1 is a schematic diagram showing a data modulating in a transmission portion of the conventional OFDM system, and FIG. 2 is a schematic diagram showing a process for adding the guard interval to the transmission portion of the conventional OFDM system. In order to avoid the inter-symbol interference and inter-channel interference resulted from the insertion of the guard interval, as shown in FIG. 2, a rear guard interval which is transformed by inverse fast fourier transform (IFFT) is reproduced and added into the front.
FIG. 3 is a schematic diagram showing an example. In this example, symbol is received from the channel with two paths, and the maximum delay length of the channel is shorter than the guard interval, and it is illustrated that signals pass through the channel when the maximum delay length of the channel is shorter than the guard interval. In order to illustrate, it is assumed that the channel has two paths.
As shown in FIG. 3, when transmitting the continuous OFDM symbol, the information of the previous symbol which delays due to the two paths is added to a portion A in the received symbol, and thus the portion A is the portion with inter-symbol interference. However, portions B and C have the same symbol information due to the guard interval. Therefore, if a starting position of FFT is arranged in the portions B to overlap the square FFT window, and then FFT is implemented, the inter-symbol interference and inter-channel interference can be prevented.
In addition to the use of preventing the inter-symbol interference and inter-channel interference, the guard interval can be used for determining the maximum delay length of the channel, so as to reduce noise. As shown in FIG. 3, the intervals A and B in the received symbol are the guard intervals, i.e. the rear portions of the reproduced symbol. Although the interval A is damaged due to the previous symbol, the interval B is completely undamaged. If the maximum delay length of the channel can be determined, the width of the interval B can be determined. Similar to the interval B which is the undamaged guard interval, the rear portion of the symbol has the same information as the original state. Therefore, if they are summed and then simply divided by 2, the signal in original state can be obtained, and the noise power can be halved, thereby improving the system performance.
In the way of using the undamaged guard interval, when the guard interval is elongated and the maximum delay length of the channel is shortened, the interval for reducing noise can be broadened, thereby improving the performance. However, when the maximum delay length of the channel is elongated, the performance gain is gradually reduced. In addition, when the maximum delay length of the channel is longer than the guard interval, the inter-symbol interference (ISI) and inter-channel interference (ICI) occur, resulting in a severe performance degradation.